1. Field of the Invention
The present invention relates to a surface acoustic wave element used in, e.g., a high-frequency filter and, more particularly, a surface acoustic wave element using diamond and a method of manufacturing the same.
2. Related Background Art
A surface acoustic wave element is an electro-mechanical conversion element utilizing a surface wave propagating on the surface of an elastic body and has an interdigital transducer (IDT) electrodes on a piezoelectric body. When an electrical signal is applied to one IDT, the piezoelectric body is stressed, and this stress becomes a surface acoustic wave. The surface acoustic wave propagates on the piezoelectric body and is extracted as an electrical signal at the other IDT. In this manner, in the surface acoustic wave element, the piezoelectric phenomenon of the piezoelectric body is used in exciting the surface acoustic wave.
The frequency characteristics of the surface acoustic wave element are given as band passing characteristics having, as a center frequency, a frequency f.sub.0 proportional to a ratio V/.lambda..sub.0 of a propagation velocity V of the surface acoustic wave to a wavelength .lambda..sub.0 of the segments of each IDT.
A surface acoustic wave element requires a smaller number of parts and can be made compact. In addition, signals can be easily input and output on a surface acoustic wave propagation path. This element can be used in a variety of applications such as a filter, a delay line, an oscillator, a convolver, or a correlator.
In particular, a surface acoustic wave filter has been used as an IF television filter from the early period of its application. Such surface acoustic wave filters have also been applied in VTRs and various communication equipment filters.
This surface acoustic filter has been manufactured such that an interdigital transducer is formed on a monocrystalline piezoelectric material such as LiNbO.sub.3 or LiTaO.sub.3. A surface acoustic wave element having a piezoelectric thin film made of ZnO or the like sputtered on a substrate as of glass is also used.
In recent years, an element used at a higher frequency is required in a surface acoustic wave filter used in the fields of mobile communications and the like. As described above, when the electrode wavelength .lambda..sub.0 decreases or the surface acoustic wave velocity V increases, the frequency characteristics of the element have a higher center frequency f.sub.0.
Under the above circumstances, a surface acoustic wave element having a piezoelectric film made of sapphire or diamond which is a material for increasing the velocity of the surface acoustic wave has been developed (e.g., Japanese Patent Laid-Open Nos. 54-38874 and 64-62911.
The sonic speed in diamond is the highest amongst all the materials and is thermally and chemically stable. For this reason, diamond has received a great deal of attention as a substrate for forming a surface acoustic wave element. Of all the surface acoustic wave elements using diamond, a surface acoustic wave element in which a diamond thin film is formed on a substrate and a piezoelectric thin film is formed on this diamond thin film is mainly examined in favor of productivity and cost.
FIG. 14 is a schematic over view showing a conventional surface acoustic wave element using diamond.
Referring to FIG. 14, this surface acoustic wave element 30 has a substrate 31 made of, e.g., silicon, a diamond thin film 32 formed on the substrate 31, IDT 36 and 37 patterned by etching a metal thin film on the diamond thin film 32, and a piezoelectric layer 38, e.g., a ZnO film formed on the surface of the diamond thin film 32 on which the IDT 36 and 37 are formed. The IDT 36 and 37 generate a surface acoustic wave.
A conventional material for the IDT 36 and 37 preferably has a low resistivity and is exemplified by Au, Ag, Al, Ti, W, Mo, or a combination of at least two metals such that Al is formed on Ti. Of these materials, aluminum has been generally used as the material for the IDT material because the electrodes can be easily formed, and because aluminum has a low specific gravity, can minimize the electric load mass effect, and has a high electric conductivity.
A material obtained by adding an additive such as Ti, Ni, Mg, or Pd having good anti-migration properties to aluminum is used as the material for the IDT to obtain a surface acoustic wave element (e.g., Japanese Patent Laid-Open No. 3-40510).
The IDT are manufactured by the following conventional process.
An electrode metal is uniformly formed on the surface of a diamond thin film by a evaporation method, or a sputtering method. A resist is uniformly applied to the surface of the electrode metal, and a mask obtained by forming an electrode pattern on a transparent flat plate made of glass or the like is placed on the resist film. The resist film is exposed with a mercury lamp or the like to form a resist pattern. The resist pattern may be formed by an electron beam method. Using the resist having the desired electrode resist pattern, a wet etching method or an RIE (Reactive Ion Etching) method is used to form comb-like electrodes.
A surface acoustic wave element having the conventional aluminum electrodes, however, is easily degraded, and the aluminum electrodes are easily failured. In particular, when the IDT are micropatterned on the order of submicrons, such failures tend to occur due to the stress by the substrate and/or the thin film and piezoelectric distortions. The disconnections of the IDT cause a defective surface acoustic wave element. As a result, reliability of the surface acoustic wave element is degraded.
The conventional aluminum electrodes are made of polycrystalline aluminum in which crystal grains are crystallographically nonuniformly oriented. The lattice constant of diamond is about 3,567 .ANG., and the lattice constant of Al is about 4,050 .ANG..
The lattice mismatching ratio of Al to diamond is 13.5%. In this manner, the lattice mismatching ratio of A1 to diamond is so high that aluminum epitaxially grown on diamond becomes polycrystalline aluminum which has epitaxial defects, epitaxial damage, abnormally grown crystal grains, and nonuniformly oriented crystal grains. In an A1 electrode consisting of polycrystalline aluminum having nonuniformly oriented crystal grains, slip occurs in the plane (111). Voids are formed in nonuniform crystal grains. When these voids grow, interconnection failures may occur.
When a signal having a high voltage level is applied to the Al electrode consisting of the polycrystalline aluminum having the nonuniformly oriented crystal grains, a strong stress acts on the aluminum electrode from the surface acoustic wave, thereby causing stress migration. When the stress migration has occurred, electrical short-circuiting, an increase in propagation loss, and characteristic degradation such as a decrease in Q value of the surface acoustic wave resonator (SAW resonator) occur. The aluminum electrodes are also degraded by stress migration caused by grain boundary diffusion.
In the surface acoustic wave element described in Japanese Patent Laid-Open No. 3-40510, although the anti-migration characteristics are improved, this improvement is not satisfactory because an alloy mainly containing aluminum is used to form the electrodes of the surface acoustic wave element and uniform or almost uniform crystal grains of the electrode material cannot be formed on the surface of the diamond layer.
In the conventional method of manufacturing a surface acoustic wave element, it is often difficult to form electrodes in the form of a micropatterned wiring by etching.